1. Field of the Invention
The present invention relates to a method of manufacturing a negative electrode for a nonaqueous electrolyte secondary battery.
2. Description of Related Art
Japanese Patent Application Publication No. 2005-135856 (JP 2005-135856 A) discloses a copper (Cu) foil having a percentage elongation of 13% or higher as a negative electrode current collector for a nonaqueous electrolyte secondary battery.
In general, a negative electrode for a nonaqueous electrolyte secondary battery is manufactured by preparing a negative electrode coating material (also referred to as “slurry” or “paste”), which contains a negative electrode active material, a thickener, a binder, and a solvent, and applying the negative electrode coating material to a copper foil to form a negative electrode mixture layer. In the related art, in the negative electrode mixture layer formed using this method, a phenomenon called binder migration is observed, and this phenomenon may affect cycle durability.
Hereinafter, a potential effect of binder migration on cycle durability will be described. In a drying step, when the solvent contained in the negative electrode coating material is volatilized, the binder contained in the negative electrode coating material migrates to the surface of the coating film together with the solvent, and then segregates on the surface. As a result, on the surface side (upper layer) of the negative electrode mixture layer, the binder (typically, synthetic rubber), which is a resistance component, is abundantly present, and thus the movement of lithium ions (Li+) is inhibited, which causes an increase in resistance. On the other hand, on the copper foil side (lower layer) of the negative electrode mixture layer, the binder is insufficient, and thus a defect such as the peeling of a part of the negative electrode mixture layer from the copper foil is likely to occur. Further, on the lower layer, since the amount of the binder which is a resistance component is small, the reactivity of the negative electrode active material increases, and the expansion and shrinkage of the negative electrode active material caused by charging and discharging becomes more severe than on the upper layer. Therefore, the copper foil cannot withstand the expansion and shrinkage of the negative electrode active material, and the peeling between the negative electrode mixture layer and the copper foil is promoted. Further, a difference in expansion or shrinkage amount between the upper layer and the lower layer causes an electrolytic solution to be non-uniformly distributed in the negative electrode mixture layer. As a result, in an in-plane direction or thickness direction of the negative electrode mixture layer, there is a variation in the reactivity of the negative electrode active material, local deterioration is promoted, and cycle durability decreases. This tendency is particularly significant when high-rate (high-current) charging and discharging is repeated.
Binder migration can be improved to some extent, for example, by decreasing the drying rate of the coating film. However, productivity decreases due to a decrease in the drying rate. Further, due to a long period of heat treatment, the thickener contained in the negative electrode mixture layer is carbonized, and thus resistance may increase.
According to JP 2005-135856 A, when the copper foil having a high percentage elongation is used, the copper foil can withstand the expansion and shrinkage of the negative electrode active material caused by charging and discharging, a defect such as the peeling of the negative electrode active material from the copper foil can be suppressed. However, the copper foil having a high percentage elongation (that is, easy to modify) is difficult to handle in an application step and causes, for example, a decrease in the dimension accuracy of the negative electrode mixture layer or a decrease in yield. The use of the copper foil itself does not suppress binder migration.